This invention relates to vehicle detectors used to detect the presence or absence of a motor vehicle in an inductive loop embedded in a roadbed. More particularly, this invention relates to a vehicle detector with a reference information saving feature upon power failure.
Vehicle detectors have been used for a substantial period of time to generate information specifying the presence or absence of a vehicle at a particular location. Such detectors have been used at vehicular traffic intersections, for example, to supply information used to control the operation of the traffic signal heads; have been used to supply control information used in conjunction with automatic entrance and exit gates in parking lots, garages and buildings; have been used in railway installations for railway car detection and control; and have been used in security barrier installations to prevent the sudden erection of a security barrier from underneath an overlying vehicle. A widely used type of vehicle detector employs the principle of period shift measurement in order to determine the presence or absence of a vehicle in or adjacent the inductive loop mounted on or in a roadbed. In such systems, a first oscillator, which typically operates in the range from about 10 to about 120 kHz is used to produce a periodic signal in a vehicle detector loop. A second oscillator operating at a much higher frequency is commonly used to generate a sample count signal over a selectable number of loop cycles. The relatively high frequency count signal is typically used to increment a counter, which stores a number corresponding to the sample count at the end of the selected number of loop cycles. This sample count is compared with a reference count stored in another counter and representative of a previous count over the same number of loop cycles in order to determine whether a vehicle has entered or departed the region of the loop in the time period between the previous sample count and the present sample count. The number of loop cycles selected is related to the sensitivity of the vehicle detector, and this number is typically set manually by a field service technician when installing or re-initializing the detector. In some detectors, this selection process is aided by an automatic default setting built into the detector system.
The initial reference value is obtained from one or more initial sample counts and stored in a reference counter. Thereafter, successive sample counts are obtained on a periodic basis, and compared with the reference count. If the two values are essentially equal, the condition of the loop remains unchanged, i.e., a vehicle has not entered or departed the loop. However, if the two numbers differ by at least a threshold amount in a first direction (termed the Call direction), the condition of the loop has changed and may signify that a vehicle has entered the loop. More specifically, in a system in which the sample count has decreased and the sample count has a numerical value less than the reference count by at least a threshold magnitude, this change signifies that the period of the loop signal has decreased (since fewer counts were accumulated during the fixed number of loop cycles), which in turn indicates that the frequency of the loop signal has increased, usually due to the presence of a vehicle in or near the loop. When these conditions exist, the vehicle detector generates a signal termed a Call Signal indicating the presence of a vehicle in the loop.
Correspondingly, if the difference between a sample count and the reference count is greater than a second threshold amount, this condition indicates that a vehicle which was formerly located in or near the loop has left the vicinity. When this condition occurs, a previously generated Call Signal is dropped.
In order to function properly, the initial reference value must be obtained while the loop is not under the influence of a vehicle. Past detectors obtain the initial reference count value by seeking the largest count obtained during the sample count process and using that number for the reference count value. Since the largest count value occurs when no vehicle is present over the loop, the detector cannot operate properly until the detector experiences the first vacant loop condition. The Call signals generated by a vehicle detector are used in a number of ways. Firstly, the Call signals are presented to an output terminal of the vehicle detector and forwarded to various types of traffic signal supervisory equipment for use in a variety of ways, depending on the system application. In addition, the Call signals are used locally to drive a visual indicator, typically a discrete light emitting diode (LED) or a multiple LED display or a liquid crystal display (LCD) to indicate the Call status of the vehicle detector, i.e. whether or not the vehicle detector is currently generating a Call signal.
Vehicle detectors with the Call signal generating capability described above are used in a wide variety of applications, including vehicle counting along a roadway or through a parking entrance or exit, vehicle speed between preselected points along a roadway, vehicle presence at an intersection controlled by a traffic control light system, in a parking installation entrance gate, in a parking stall, in railroad yards and numerous other applications.
Most present day vehicle detectors are designed and manufactured using a microprocessor-based architecture. This type of system architecture uses volatile random access memory (RAM) to store reference samples, status information and other information (such as system sensitivity) needed for the proper identification of vehicle arrival or departure from the location monitored by the vehicle detector. Such vehicle detectors are sensitive and vulnerable to power outages, particularly due to the severe environment in which they are typically installed. When power to a detector is interrupted, the reference information stored in the volatile system RAM is lost. When power is subsequently restored, the vehicle detector must resume operation without the lost reference information. This can lead to improper and dangerous operating conditions, as the following examples will demonstrate.
In parking lot entrance installations, a vehicle detector is commonly used to provide advisory signals used in the operation of an automatic gate. When a vehicle enters a loop in front of the gate, the vehicle detector normally generates a Call signal, which is used to open the gate so that the vehicle can pass through the gated entrance and proceed to a parking stall. Once the vehicle has left the loop, the vehicle detector drops the Call signal and the gate is operated to the closed position. If a power outage occurs when the vehicle is over the loop and the reference information is lost from system RAM, the current reference value is lost. If the vehicle is still over the loop when power is resumed, a new reference value is obtained which prevents the detection of the continued presence of the vehicle over the loop. As a result, the automatic gate is closed while the vehicle is in the gate area, usually damaging the vehicle.
In rail yard applications, vehicle detectors are commonly used to help control the operation of track switches. More particularly, in such installations the need frequently arises to move rail cars from one track to another for traffic management purposes. In order to move a rail car from one track to another, the car is propelled along the present track toward a track switch. Before the railcar reaches the track switch, the switch is operated to divert the approaching car from the present track to a desired different track. The vehicle detector is usually connected to a loop positioned to monitor the track switch region in order to detect the presence of a rail car over the switch region. If a rail car is present over the switch region, the Call signal generated by the vehicle detector is used to prevent operation of the track switch in order to preclude derailing of the rail car. If a power outage occurs while a rail car is present over the switch region and the reference information is lost from system RAM, the current reference value is lost. If the rail car is still over the loop when power is resumed, a new reference value is obtained which prevents the detection of the continued presence of the rail car over the loop in the track switch region. As a result, if the track switch is operated the rail car can be derailed.
In a left turn lane at a controlled vehicle intersection, a vehicle detector is typically used to monitor the presence of a vehicle waiting for the control green signal (commonly a left-pointing arrow) so that the vehicle can proceed into the intersection and make a left turn. If no vehicle is detected at the time the left turn signal is normally turned green by the intersection controller, this phase of the traffic control cycle is typically skipped, so that the left turn signal remains red. If a vehicle is detected, the left turn phase is entered and the vehicle is given a green signal thereby permitting the vehicle to proceed into the intersection a make a left turn. If a power outage occurs while a vehicle is present over the left-turn loop and the reference information is lost from system RAM, the current reference value is lost. If the vehicle is still over the left-turn loop when power is resumed, a new reference value is obtained which prevents the detection of the continued presence of the vehicle waiting for the left turn signal to turn green. Since the vehicle waiting for the green left turn arrow is no longer detected following an interruption of power to the vehicle detector and traffic signal controller, the traffic signal controller provides a safe condition for the intersection upon the return of power. The traffic signal controller accomplishes a safe start up condition, following the return of power, by providing a minimum amount of green time for each traffic lane. This minimum amount of green time ensures that traffic in each lane is allowed to move so that each detector experiences, as a minimum, a momentary vacant condition; and therefore is able to obtain a valid reference count value. Without the traffic signal controller providing green time for each traffic lane following the application of power, the detectors would not be able to obtain a valid reference value; thus creating a very dangerous condition, which could result in an accident. In security barrier applications, vehicle detectors are used to condition the operation of retractable barrier posts or solid structures designed to prevent entry of vehicles into a secure area. Such barrier devices are typically designed to be in a normally erect position above the surface of the ground or pavement. Normally, an approaching vehicle encounters the erect barrier and cannot enter the area unless authorized to do so by a human operator (e.g. a security guard posted at the entrance to the area) or an automatic authorization system (such as a card-actuated barrier operating system). A vehicle detector system is typically installed in a position to generate a call signal whenever a vehicle is positioned over the barrier when in the retracted state in order to prevent the erection of the barrier from beneath the vehicle and consequent damage. If a power outage occurs when a vehicle is over a retracted barrier and the reference information is lost from system RAM, the current reference is lost. If the vehicle is still over the loop (and thus the barrier) when power is resumed, a new reference value is obtained which prevents the detection of the continued presence of the vehicle over the loop. As a result, the barrier is suddenly erected and the vehicle is usually severely damaged.
As will now be apparent, the need exists for some mechanism to prevent the loss of reference information in a vehicle detector when a power outage occurs. In a microprocessor-based vehicle detector, a workable solution might appear to be to add non-volatile memory and store the reference information in this memory each time the values are updated. However, this approach is not practically feasible. Known non-volatile memory devices suffer from the limitation of possessing only a finite number of read/write cycles. After this limit has been reached, a typical non-volatile memory device cannot be relied upon to reliably store and retrieve information. The limit is typically about one million erase/write cycles, after which the device manufacturer will no longer guarantee reliability. In a typical microprocessor-based vehicle detector, the number of samples taken per second can be as high as one thousand, depending on the sensitivity setting of the detector (the sensitivity setting establishes the length of the sample period, usually defined by the number of loop cycles during which the high speed counter is permitted to accumulate counts). Consequently, the reliability limit, and thus the useful lifetime, of a non-volatile memory device in such a vehicle detector can be reached after only one thousand seconds of operation, or slightly less than seventeen minutes, if the detector is being operated at the lowest sensitivity. Even at higher sensitivities, the useful lifetime of a non-volatile memory device in a vehicle detector can be exceeded in less than forty hours of operation. Since vehicle detectors are expected to operate reliably in situ for years, the simple addition of non-volatile storage to permanently store reference information is not a practical solution to the problem.